Plate lifting clamp with automatic release mechanism



April 1970 E. M. GARDNER 3,507,534

PLATE LIFTING CLAMP WITH AUTOMATIC RELEASE MECHANISM Filed Dec. 30, 1968 INVENTOR EDWARD MERRILL GARDNER m mwg rmzw ATTORNEYS United States Patent Int. Cl. B66c 1/24 US. Cl. 294-104 12 Claims ABSTRACT OF THE DISCLOSURE The plate-lifting clamp disclosed comprises mechanism including a cam lever pivoted to the body of the clamp and means pivotally connecting the lever with the peripheral portion of the radius link of the clamp for actuating and forcing the gripping cam to a plate-engaging position in tthe plate-receiving space and to a retracted position within the body of the clamp. In this construction the means connecting the peripheral portion of the radius link with the cam of the cam lever includes a rigid elongated barrel pivotally connected to tthe radius link, a compression spring in the barrel bearing on its bottom, a rod pivoted to the cam lever extending through the bottom of the barrel and the compression spring and connected to the upper part of the compression spring to hold the spring under compression. The gripping cam is releasably held in retracted position by the frictional engagement of the bottom of the barrel with the camming surface of the cam lever, under the action of the compression spring.

CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of the applicants pending application Ser. No. 641,295, filed May 25, 1967, and allowed Oct. 28, 1968.

This pending application discloses an improvement over prior clamp structures by including a spring-biased gripping cam actuating mechanism provided with an actuating lever for moving the gripping cam to either the plategripping position or to a retracted position. The clamp also includes a readily releasable latching means for holding the lever and gripping cam in a retracted position, from which it is readily releasable by the application of a lifting force on the radius link to which the cam is connected.

BACKGROUND OF THE INVENTION Field of the invention The invention of this application relates to an improve ment in steel plate gripping and lifting clamps of the type in which the gripping cam is actuated to gripping and retracted positions by a spring biased leverage mechanism.

Description of the prior art Known clamps of the type referred to include a generally U-shaped structure providing a plate-receiving space between opposed jaws open at the projecting ends thereof for receiving a steel plate. These clamps include a movable plate gripping cam pivoted in one of the jaws and facing toward the other jaw, also a leverage mechanism located in the body of the clamp, including a radius link fulcrumed on a pivot pin and spaced from the gripping cam. In this construction, one end of the radius link is connected to a lifting shackle, while the other end is connected by a link with the gripping cam at a point intermediate its pivot axis and its outer end. It is known that in such clamp constructions spring biased mechanisms may be provided for actuating or biasing the gripping cam to either a plate gripping or to a retracted position.

SUMMARY OF THE INVENTION The present invention relates to improvements in clamps for lifting steel plates, such clamps being generally of the type including a heavy forged steel body provided with a plate-receiving slot across which a pivoted gripping cam is swung into engagement with the plate to be lifted. This cam preferably faces a gripping pad across the slot, and is connected to a lifting linkage including a lifting shackle, a radius link and a link connecting the radius link with the cam.

In accordance with the present improvements, the clamp is provided with a cam-actuating and locking mechanism for releasably retaining the gripping cam in a retracted position and for locking it in a plate-engaging position, such mechanism comprising a cam lever unit and spring assembly arranged so that the spring is always under compression. The cam lever is located in back of the gripping cam, pivoted to the body of the clamp and joined by a connecting means with the peripheral portion of the radius link. The connecting means is pivoted to the cam lever and includes means frictionally engaging the camming surface of the cam lever when the gripping cam is in retracted position and being moved to and from that position.

When the cam lever is swung to an upper position, force is applied to the radius link to swing the gripping cam to its retracted position. When the cam lever is swung around through more or less, a pulling force is applied to the radius link to actuate the gripping cam to a plate-engaging position and the cam lever and its connecting means are locked in over dead center.

In a preferred construction the connecting means comprises an elongated barrel housing the compression spring. The upper end portion of the barrel is pivoted to the peripheral portion of the radius link while the bottom of the barrel has a bore. The connecting means also includes a rod pivoted to the rotatable cam, extending through the bore in the barrel and through the compression spring. The upper end of the rod is connected to the upper end of the spring to keep it in compression. The cam lever is provided with an arcuate camming surface engaged by the bottom end of the barrel to provide friction necessary to hold the gripping cam releasably in retracted position.

The main object of the invention is to provide cam lever-actuated mechanism for a plate-lifting clamp which is more effective than the actuating mechanism of the pending application and which avoids difiiculties encountered with tension springs of prior clamps and the dangers encountered from slippage, breakage of springs and the failure of an operator to apply the actuating lever to grip a plate.

A further object of the present improvement is to provide a new cam-actuating mechanism with cam lever friction hold and release for the gripping cam in retracted position.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention is described more in detail hereinafter in connection with the accompanying drawing illustrating a single improved embodiment and forming a part of this application.

In the drawing:

FIG. 1 is an elevational view with parts in section with the front side plate removed to show the internal structure of the clamp with the gripping cam in gripping position; and

FIG. 2 is a view similar to that of FIG. 1, showing the gripping cam in retracted position with the cam lever actuating mechanism frictionally holding the radius link and gripping cam.

DESCRIPTION OF PREFERRED EMBODIMENT Referring to the figures of the drawing, the platelifting clamp shown therein comprises a single embodiment including the combinations and improvements of the present application. The clamp comprises a clamp body 8 in the form of a U-shaped structure including jaws 10 and 12, the latter of which is shorter than the former. These jaws face each other to provide a space or elongated slot 14 in which a plate to be gripped and lifted is received. The short jaw 12 is provided with a serrated gripping pad 16, of a known type, cooperating with a gripping cam also of known type pivotally mounted in the jaw 10.

The body 8 of the clamp is made up of a pair of similar spaced face plates 22, the front plate of which has been removed, held in spaced relation by some of the interior mechanisms, such as the end pins of the radius link 36 and the hubs of the gripping cam 20, and secured together by rivets, such as the rivets 26.

The gripping cam 20 is approximately located between face plates 22 in the longer jaw 10 and pivotally mounted on a pin 28 extending through both plates 22. The cam 20 includes hub portions 30 which aid in spacing the plates 22.

The clamp is provided with a lifting shackle 32 pivoted by a pin 34 to one end of a radius link or lever 36 which in turn is fulcrumed on a pivot pin 38 extending through the plates 22 and located in a position above and to the left of the space 14. The other end of the radius link 36 carries a pivot pin 40 connecting one end of a link 41 comprised of two similar parallel members on the respective sides of the radius link 36 and cam 20.- The other end of the link 41 is pivoted to the cam 20 on a pin 44, at a point intermediate the tip of the cam and its pivot pin 28.

The improved cam-actuated and locking mechanism of the present application comprises an elongated compression spring 46 located in an elongated barrel 48 around a rod 50 extending through and movable in a bottom bore 52. The lower end of the rod 50 is pivotally connected to a cam lever 54 by a pivot pin 56. The spring 46 is held under compression, under all conditions, in the barrel 48 by a nut 58 threaded on the upper end of the rod 50. This nut 58 is slidable in the barrel 48 as the compression of the spring 46 is increased and decreased during operation of the gripping cam-actuating and locking mechanism.

The barrel 48 as well as the portion of the cam lever 54 carrying the pivot pin 56 is located generally in back of the gripping cam 20 and slightly to one side, forward in the drawing, so that it can move past the radius link 36 and the link 41, as indicated in FIG. 2. A lug 60 is provided on the side of the barrel 48 near its upper end, and connected by a pivot pin 62 with a forwardly projecting lug or mounting 64 located on the face of the radius link 36. The mounting 64 extends parallel to the pin 38 a distance suflicient to permit the barrel 48 to move past the pin 40 and link 41, as shown in FIG. 2. Furthermore, the mounting 64 is located near the periphery of the radius link 36 above the pin 40 in FIG. 1, and approximately in line with the pins 34 and 38, when the relationship of the cam 20, link 41 and radius link 36 is approximately as shown in FIGS. 1 and 2.

The lever 54 comprises a cam 68 integral with a handle 66 attached to the far side and lower end portion of the cam. The cam 68 comprises two spaced integral plates respectively on the opposite sides of the rod 50. They are respectively attached to the clamp plates 22 by axially aligned pins 70, only one of which is shown in the drawing. The cam 68 like the barrel 48 is located in a position partly in front of the cam 20, as indicated 4 in FIG. 2. The handle 66 may be operated by the use of a strap 67 attached through a hole in the end portion of the handle.

When the gripping cam 20 is in the position shown in FIG. 1, or in contact with a steel plate or beam to be lifted, the lever 54 is used to apply the force of the spring 46 for holding the cam 20 in engagement with the plate. In this position the handle 66 engages a pin 72, in the back plate 22, located in a position such that the pin 56 in the cam portion 68 is moved to an overcenter locking position with respect to a line through the axes of the pins 62 and 70. In this position the spring 46 applies a strong force tending to rotate the radius link 36 in a counterclockwise direction and press the cam 20 against a beam or plate to be or being lifted. If there is any force, such as the thickness of the plate, which would cause rotation of the radius link 36 in a clockwise direction, that would serve to increase the compression of the spring 46.

FIG. 2 illustrates the position of the cam lever actuating and locking mechanism when it is desired to releasably hold the gripping cam 20 in retracted position, as shown. The cam 20 may be actuated to this position by swinging the handle 66 around to a position where it engages a pin or rivet 74 in the back plate 22. In pivoting the lever around from the position shown in FIG. 1 to that shown in FIG. 2, the cam surfaces 69 are brought into engagement with the lower end of the barrel 48 to apply a positive force rotating the radius link 36 in a clockwise direction and, accordingly, retracting the cam 20 into the body of the clamp, so that it does not project into the opening 14.

The cam lever 54, in the position shown in FIG. 2, is not in an overcenter position with respect to a line through the axes of the pins 62 and 70 and is not positively locked for retaining the gripping cam 20 in retracted position. However, the point of engagement P of the bottom of the barrel 48 with the arc of the cam surfaces 69 is in an overcenter position when locked open.

The reason for this improved construction and arrangement is that the cam-actuating and locking mechanism in the position shown in FIG. 2 is such as to hold the weight of the clamp with the cam 20 in retracted position. However, the compression spring unit including the rod 50 is connected to the lever 54 at a point, 56, where it never goes over deadcenter, and, therefore, when the clamp is placed on a horizontally positioned plate and a pull is applied to the lifting shackle 32 and the radius link 36, tending to rotate the latter counterclockwise, the resulting force is suflicient to overpower the internal forces created by the over toggling of the cam 68 against the bottom of barrel 48, and the lever 54 is rotated counterclockwise on the pivot pins 70 to break the overlooked position and allow the spring 76 to activate the gipping cam 20.

During this movement of the cam lever, the cam surface 69 slides along the lower end of the barrel 48 resisted by friction and a clockwise torque about pin 56. Since the bottom of the barrel 48 bears on the cam surfaces 69, there is a clockwise torque and a frictional holding of cam 68 which in turn holds cam 20 in retracted position until overcome by a pull on the lifting shackle 32.

Locking by spring 76, as in FIG. 1, is automatically applied if the operator puts the clamp on a plate, for example in horizontal position, and fails to move the lever 54 to the overcenter locking position. This locking is achieved by the spring 76 which is released as soon as lockedopen position is overcome by the pull of the shackle 32 As stated above, springs are known for applying a continuous bias to gripping cams, but as shown in the drawing, a spring 76 is coiled around the cam hub 30 with one end hooked around a pin 78 on the front face of the cam 20 as shown, while the opposite end of the spring is hooked around a rivet 80 projecting inwardly from the front face plate 22. Since the cam 20 along with the radius link 36 are set almost against the back plate 22, the hub 30 is rather long so that the spring 76 usually comprises about six turns around the hub extending from the face of the cam 20 to the inner face of the front plate 22. The spring 76 applies bias tending to move the cam 20 toward gripping position, but this bias is not sufficiently strong, to overcome the friction of the barrel 48 against the cam surfaces 69 and the clockwise torque due to over toggling and cause rotation of the lever 54 from the position shown in FIG. 2, even though the spring 76 is under the greatest strain in FIG. 2.

When there is a pull on the lifting shackle 32 by the weight of the clamp alone hanging on a hook with the cam lever actuating mechanism in the position shown in FIG. 2, it has a tendency to rotate the radius link 36 in a counter clockwise direction and a tendency to unloosen the locked open mechanism and force the cam 20 to a closed position. The spring 76 has this tendency also, but there is no movement until an additional load pulls the clamp down such as starting to lift a horizontally positioned plate causing the lock-open position to no longer be maintained, and the clamp functions in its usual manner through linkage pressure on cam 20 to hold the load and in addition to be locked automatically as a safety measure to overcome indifferent or careless operators.

The mechanics of this unique safety feature may be simply explained as follows: The point of contact P, FIG. 2, is over dead center. More specifically, it is to the right of the point where a line drawn from the center of pin 56 and perpendicular to the lower end surface of member 48 intersects that surface. This line is shorter than the length of the cam from the center of pin 56 to the point P. Therefore, cam 68 is mechanically locked by the strong spring 46 because a counter clockwise rotation of cam 68 must of necessity further compress spring 46 until the cam 68 goes over dead center. Friction of moving parts also helps to hold it locked. There is an internal torque created by spring 46 tending to rotate cam 68 clockwise about pin 70. This torque is limited and only just strong enough to resist the torque in the opposite direction created by the spring 76 acting through the linkages, and the pull of the weight of the clamp on shackle 32. Since the point P is at left of a line between pin 62 and pin 70, these forces acting downward on pin 62 tend to rotate cam 68 in counter clockwise direction. So, as soon as a load is lifted, the counter clockwise torque overpowers the friction and the clockwise torque on cam 68 and compresses spring 46 causing cam 68 to move over dead center and release with a snap so that lever 66 is forced to the stop 72, as shown in FIG. 1 when the clamp is on thick plates. The clamp is then in operating position for lifting with a double lock by springs 46 and 76 and the linkage pull in proportion to the load lifted. On lifting a horizontally located plate, this automatic lock prevents the plate from dropping out if an operator forgets to take off the lock-open device.

When a plate to be lifted is received into the slot 14 as the clamp is placed on the plate, the handle 66 of the locking lever is then moved from the position shown in FIG. 2 to that shown in FIG. 1. The compression spring 46 and the spring 76 force the gripping cam 20 against the plate and the clamp onto the plate. The locking mechanism is locked over dead center aided by spring 76 to hold the cam 20 on the plate as soon as the handle 66 engages the rivet 72. After a steel plate is locked in the slot 14, a lifting force is applied to the shackle 32, and that increases the pressure of the cam 20 against the plate.

I claim:

1. In a clamp of the type used for lifting steel plates, beams and the like including a body having spaced opposed projecting jaws defining a U-shaped structure providing a plate-receiving space between the jaws open at the projecting ends thereof for receiving a steel plate to be gripped and lifted by the clamp:

(a) a plate-gripping means on one of said jaws facing the other jaw across said space:

(b) a plate-gripping cam pivoted in said other jaw adjacent to said space and and having a gripping surface facing toward said gripping means,

(c) a leverage mechanism located in the body of the clamp including a radius link fulcrumed on a pivot pin in the body of the clamp spaced from said gripping cam,

(d) a lifting connector connected to one end of said radius link,

(e) a link connecting the other end of the radius link with the gripping cam at a point intermediate its pivot axis and its outer end,

wherein the improvement comprises:

(f) mechanism including a cam lever pivoted to the body of the clamp and means pivotally connecting the cam lever with a peripheral portion of the radius link for actuating and forcing the gripping cam to a plate engaging position in said receiving space and to a retracted position within the body of the clamp, and wherein (g) the cam lever is provided with a camming surface and the means connecting the cam lever with the radius link includes a rigid member frictionally en gaging said camming surface when the gripping cam is in retracted position.

2. A clamp as claimed in claim 1, wherein the rigid member is a tubular barrel, and a compression spring located in the barrel under compression and biasing the barrel toward and in contact with the camming surface of the cam.

3. A clamp as claimed in claim 2, wherein the upper end of the barrel is pivoted to the radius link in a position such that it extends across the radius link and said link when the gripping cam is in retracted position.

4. A clamp as claimed in claim 1, wherein the means connecting the cam lever with the radius link is pivoted to the upper part of the radius link between its end connections.

5. A clamp as claimed in claim 1, wherein the frictional engagement between the rigid member and the camming surface applies sufficient resistance to movement to retain the gripping cam in retracted position, which resistance may be automatically overcome by the application of a lifting force on the radius link when the clamp is in position on a plate to be lifted.

6. A clamp as claimed in claim 1, wherein the engagement point between the rigid member and the camming surface of the cam lever is over the dead center point which may be described as the point on the end surface of rigid member which is the shortest distance from the center of pin 56.

7. A clamp as claimed in claim 6, wherein the above mentioned engagement point does not reach the dead center of a straight line between the two points where the cam lever is pivoted to the body of the clamp and where the radius link and connecting means are pivoted to each other.

=8. A clamp as claimed in claim 1, wherein when the clamp is hanging in a locked-open position, there is a means applying an internal spring motivated torque opposing the closing of the cam, but when an additional pull is exerted on the lifting shackle there is means applying sufficient torque to overcome the internal locking and frictional forces.

9. In a clamp of the type used for lifting steel plates, beams and the like including a body having spaced opposed projecting jaws defining a U-shaped structure providing a plate-receiving space between the jaws open at the projecting ends thereof for receiving a steel plate to be gripped and lifted by the clamp:

(a) a plate-gripping means on one of said jaws facing the other jaw across said space,

(b) a plate-gripping cam pivoted in said other jaw adjacent to said space and having a gripping surface facing toward said gripping means,

(c) a leverage mechanism located in the body of the clamp including a radius link fulcrumed on a pivot pin in the body of the clamp spaced from said gripping cam,

(d) a lifting connector connected to one end of said radius link,

(e) a link connecting the other end of the radius link with the gripping cam at a point intermediate its pivot axis and its outer end,

wherein the improvement comprises:

(f) mechanism including a cam lever pivoted to the body of the clamp and means pivotally connecting the cam lever with a peripheral portion of the radius link for actuating and forcing the gripping cam to a plate engaging position in said receiving space and to a retracted position within the body of the clamp, and wherein (g) the cam lever includes a cam having a convex camming surface increasing in distance from the cam lever pivot and to be frictionally engaged by the connecting means.

10. A clamp as claimed in claim 9, wherein the means connecting the cam lever with the radius link comprises an elongated tubular barrel, a compression spring located in 8 the barrel, a rod pivoted to the cam lever, extending into the end of the barrel adjacent to the cam lever and through the compression spring, and means connecting the upper end of the rod with the upper end of the compression spring for forcing the barrel toward the camming surface of the cam lever.

11. A clamp as claimed in claim 10, wherein the cam lever comprises a convex camming are increasing in distance from its pivot point, said rod being pivoted to the cam lever at a point intermediate its pivot point and the maximum distance of the arc therefrom.

12. A clamp as claimed in claim 10, wherein the cam lever comprises a connected pair of similar spaced disclike members between which said rod extends and to both of which it is pivoted on a pivot pin.

References Cited UNITED STATES PATENTS 3,071,406 1/1963 Luckner 294-104 3,273,671 9/1966 Urana 18844 3,414,315 12/1968 OQuinn 294-104 HARVEY C. 'HORNSBY, Primary Examiner U.S. Cl. X.R. 188-44 

